The present invention relates to a method of joining cellulosic fibre containing surfaces by means of a phenolic resin, phenol-resorcinol, or resorcinol resin based adhesive system whereby at least one of the surfaces is treated with a cure promoter composition prior to application of the adhesive system. It also relates to a cure promoter composition for a phenolic resin, phenol-resorcinol, or resorcinol resin based adhesive system.
A xe2x80x9ccure promoterxe2x80x9d is a substance, i.e. a Compound or a composition, which increases the curing rate of an adhesive on forming of the joint. It should be distinguished from xe2x80x9chardenersxe2x80x9d and xe2x80x9ccuring agentsxe2x80x9d, which are substances used to initiate the curing process of an adhesive system. The cure promoter is in the form of a fluid.
Phenolic resin (Chemical Abstracts Service Registry Number 9003-35-4) based and resorcinol resin (Chemical Abstracts Service Registry Number 24969-11-7) based adhesive system are described, for instance, in xe2x80x9cUllman""s Encyclopaedia of Industrial Chemistryxe2x80x9d, 5th edition, Volume A 1, page 230 (1985). Phenol-resorcinol resin such as phenol-resorcinol-formaldehyde (PRF) adhesives, are for example described in U.S. Pat. No. 4,061,620.
Resorcinol resin and phenol-resorcinol resin adhesives can generally be cured at both low and elevated temperatures, whereas phenolic resin adhesives can generally only be cured at elevated temperatures, i.e. Txe2x89xa7100xc2x0 C. The molar ratio of formaldehyde:phenol-resorcinol and formaldehyde:resorcinol (hereinafter both referred as HCHO:ArOH) is generally xe2x89xa60.80 for phenol-resorcinol and resorcinol resin adhesives, while the molar ratio of formaldehyde:phenol (hereinafter referred as HCHO:PhOH) for phenolic resin adhesives is generally xe2x89xa71.0.
By an xe2x80x9cadhesive systemxe2x80x9d is herein meant a curing formulation of a resin and a hardener or a curing agent.
A xe2x80x9ccellulosic fibre containing surfacexe2x80x9d may in the present context be a surface substantially defined by fibres of cellulose or one or more cellulose derivatives, or combinations thereof. Examples of cellulosic fibre containing surfaces are the surfaces of wood based panels, paper or paperboard. The term xe2x80x9cwood based panelxe2x80x9d comprises, inter alia, plywood, particleboards, flakeboards, fibreboards, such as hardboards and medium density fibreboards (MDF). The present invention also covers embodiments in which only a thin surface stratum contains cellulosic fibres, the stratum being applied on the surface of an object of a basically non-cellulosic nature.
EP 493,010 discloses a method belonging to the general technical field indicated in the introduction. Pieces of unseasoned wood are joined by means of an adhesive formulation, whereby a cure promoter is brought into contact with the adhesive. Formaldehyde based adhesives are said to be preferred, and phenol and resorcinol are indicated as examples of components that may be used in such adhesives. Lewis and Bronsted acids are among the cure promoters mentioned specifically organic acids of low molecular weight. Basic chemicals (pH greater than 7) which will react on contact with the adhesive to change the nature of a conventional adhesive to a fast curing adhesive are indicated to constitute a preferred class of cure promoters. Sodium periodate is also used in one of the examples.
These specific promoter chemicals indicated in EP 493,010 are all burdened by serious disadvantages, such as being poisonous, e.g. chromic acid, zinc salts, and aluminium salts, or giving rise to odour problems, e.g. ammonia, amines and amine derivatives, or both, e.g. formic acid and trichloroacetic acid. Sodium periodate is known to be a strong oxidising agent, giving rise to severe fire hazards.
It has also been found that when ammonia or an amine or an amine derivative is used as cure promoter the exact dosing of the promoter chemical is extremely critical; even at a minor overdose of the promoter the curing of the resin will be premature, as it will cure before it has penetrated the wood sufficiently to provide a durable joint between the surfaces.
The present invention accordingly provides a method of joining cellulocic fibre containing surfaces and a cure promoter composition suitable for use in the method, by which enhanced adhesive and strength properties are obtained and at the same time the above mentioned problems can be overcome.
The claimed method relates to joining of cellulosic fibre containing surfaces by means of a phenolic, phenol-resorcinol, or resorcinol resin based adhesive system, whereby at least one of the surfaces is treated with a cure promoter prior to the adhesive system being applied, in which method a cure promoter comprising a carbonate is used.
The surface treated with the cure promoter composition may, for instance, be the surface of a wood based panel or the surface of a paper or a paperboard. The wood based panel may for instance be a particleboard, flakeboard, hardboard, fibreboard, specifically a medium density fibreboard (MDF), or be made of plywood, although this listing is not exhaustive.
Suitable adhesive systems for use in the present invention are those based on phenolic, phenol-resorcinol, or resorcinol resins, and preferably those based on phenol-resorcinol, or resorcinol resins. The HCHO:ArOH molar ratio of the phenol-resorcinol and resorcinol resin component of the adhesive system is suitably 0.4:1 to 0.8:1, preferably 0.45:1 to 0.75:1 and most preferably 0.50:1 to 0.70:1. The resorcinol:phenol molar ratio of the phenol-resorcinol resin component of the adhesive system is suitably 1:0-1:1, preferably 9:1-0.15:1 and most preferably 7:1-0.25:1.
The adhesive systems used in the claimed method are cured at a temperature of suitably 10 to 100xc2x0 C., preferably 15 to 80xc2x0 C. and most preferably at about room temperature, for instance at 20 to 40xc2x0 C.
The cure promoter composition according to the present invention for a phenolic, phenol-resorcinol, or resorcinol resin based adhesive system comprises a carbonate.
The carbonate is preferably a water-soluble salt of carbonic acid, such as for instance Na2CO3, NaHCO3, K2CO3, KHCO3, (NH4)2CO3, or NH4HCO3. The also be a combination of two or more water-soluble carbonic acid salts. The cure promoter composition suitably comprises 0.5-45 weight percent water soluble carbonic acid salt, preferably 5-40 weight percent and most preferably 10-35 weight percent, based on the total weight of the cure promoter composition.
In one embodiment the carbonate is 1,3-dioxolan-2-one (Chemical Abstracts Service Registry Number 96-49-1) or a higher homolog thereof, such as for instance 4-methyl-1,3-dioxolan-2-one (Chemical Abstracts Service Registry Number 108-32-7), or a combination thereof. The carbonate may also be a combination of 1,3-dioxolan-2-one, or a higher homolog thereof, and one and more water-soluble carbonic acid salts, for instance one or more of the salts indicated above.
Preferably, the present cure promoter composition additionally comprises a base, in particular an alkali. By xe2x80x9calkalixe2x80x9d is meant water soluble hydroxides of alkali metals and alkaline earths, in particular Li, K, Na, Rb, Cs, and Ca. Preferably, the alkali is NaOH or KOH. The alkali is suitably used in an amount of up to about 30 percent by weight, based on the total weight of the cure promoter composition, preferably up to about 10 percent by weight and most preferably up to about 5 percent by weight.
The present cure promoter composition may additionally comprise a water retaining and viscosity regulating agent, such as for instance polyvinyl alcohol, hydroxy ethyl cellulose, carboxy methyl cellulose, ethylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol, or a combination thereof.
Preferably, the present cure promoter composition is substantially formaldehyde and/or resorcinol free and most preferably substantially resorcinol free. The present invention is illustrated by means of some non-limiting examples below.